Conventional methods for removing dirt from structural surface include washing with water, washing with chemical, sand-blasting, and the like. Such conventional methods have a problem in that they tend to scatter water or dirt substance to the surrounding, and it is usually difficult to prevent such scattering completely. Due to the increased public concern on environment, unless such problem is solved, chance of using the conventional methods will be gradually diminished.
To solve the above problems of the conventional methods, the inventors disclosed an invention titled "Cleaning Method Of Indoor and Outdoor Structural Surface" in his Japanese Patent Application No. 321032/1995 filed on Nov. 15, 1995. This cleaning method will be briefly reviewed by referring to FIGS. 5A-5E and 6 showing a case of cleaning the surface 10 of a building structure. In this example, cleaning zones 10a, 10b, 10c, . . . of an overall surface 10 of a building are successively cleaned one after another (see steps 601 and 602 of FIG. 6).
At first, it is confirmed that a structural wall is divided into a number of cleaning zones and that one of such zones, e.g., the cleaning zone 10a is to be cleaned to begin with, and a polymer solution 5a, which is made by dissolving adhering-membrane-forming-polymer 2a in a solvent 9, is applied to the zone 10a twice in the step 603. Each step of application produces a thin membrane 6a on the cleaning zone 10a, as shown in FIG. 5A. Arrows a and b indicate that, after a thin membrane 6a formed by a first application of the polymer 2a as shown by the arrow a is dried by evaporation of the solvent 9 to become an adhering membrane, a second application as shown by the arrow b is made so as to produce another thin membrane 6a applied thereon. With the use of two thin membranes 6a, a laminated adhering membrane 7a is formed on the cleaning zone 10a in a peelable manner, as shown in FIG. 5B. Dirt substance on the cleaning zone 10a are caused to adhere to the laminated adhering membrane 7a for cleaning the zone 10a at the step 604.
FIG. 5B through FIG. 5E show that by repetition of the steps 602-604, the remaining cleaning zones 10b, 10c, . . . of the surface 10 are also covered by the laminated adhering membranes 7a and cleaned (see steps 605 and 606 of FIG. 6). FIG. 5E indicates that, in this example, after the entire building structure is finished the laminated adhering membranes 7a on all the cleaning zones 10a, 10b, 10c, . . . of the structural surface 10 are removed simultaneously in one stroke (see step 607 of FIG. 6.)
With the cleaning method for the surface 10 of a structure, as shown in FIGS. 5A-5E and 6, a laminated adhering membrane 7a can be formed on a wide surface or intricately shaped surface of a structure in a short period of time simply by applying a polymer solution 5a thereon twice through brushing or spraying. The method not only facilitates removal of dirt substances, but also provides protection of structural surface and prevention from dirt deposit, and one can expect saving in labor for such cleaning, protection, and prevention of deposit by using the method. Examples of the adhering-membrane-forming-polymer 2a include polyvinyl alcohol, carboxymethyl cellulose, polyvinyl chloride, acrylic resin, and polyvinyl butyral. The solvent 9 can be water or an organic solvent.
Thus, the method of cleaning structural surface by using the above laminated adhering membrane 7a has certain advantages; e.g., in the ease of operation for applying the polymer solution, in the readiness of handling the polymer solution, in facilitation of peeling operation of the polymer membrane by using the laminated structure of the membrane, and in simplification of the disposal of the used membranes. If water is used as the solvent 9 of the polymer solution 5a, there is no risk of generating poisonous gas or stench gas when applying it on surfaces to be cleaned, and the solution is free from catching fire.
Membranes formed by spreading of aqueous solution of water-soluble polymer, however, tend to be weakened and lose flexibility when water contained therein evaporates to dry them, despite that as long as moisture above a certain level is kept the flexibility and toughness of the membranes are maintained. Weakened membranes are easily torn when peeling force is applied thereto, and the process of peeling the membrane becomes cumbersome and time-consuming. Especially, in the case of a rough structural surface with projections and recesses, when the aqueous polymer solution is applied thereon and a membrane is formed by drying of it, the membrane tends to become comparatively thin at portions corresponding to the projections of the rough surface and comparatively thick at portions corresponding to the recesses thereof. Due to the thickness difference at different portions of the membrane, unevenness of strength is produced therein; i.e., there are weak portions and strong portions in the membrane. When peeled from structural surface, the membrane tears at weak portions and tearing cracks spread, so that pealing of the membrane as one piece becomes difficult. Even with the above-mentioned "Cleaning Method Of Indoor and Outdoor Structural Surface", it was difficult to prevent the tearing of the laminated adhering membrane due to the roughness of the structural surface. When torn, broken pieces of the membrane tend to be scattered around the structure, and laborious process of collecting the scattered pieces and cleaning the surrounding becomes indispensable. Thus, there has been a need for solving the problem related to the weakness of the membrane of water-soluble polymer.
Therefore, it is an object of the invention to provide a method for cleaning structural surface by using easily peelable and readily recoverable membrane of water-soluble polymer.